Luminance of a pixel can be controlled in dependence on a greyscale value. A greyscale value is used for example to indicate a greyscale level of a display effect output by a pixel, ranging from a minimum luminance level (black) to a maximum luminance level (white). Each greyscale level has a corresponding luminance value. In an electrowetting cell (for example a pixel or sub-pixel), the greyscale value corresponds for example with a magnitude of a voltage applied to the electrowetting cell and a corresponding configuration of first and second fluids in the cell. For example, for an 8 bit system, 256 greyscale values can be indicated, with 0 indicating a darkest (e.g. black) greyscale level and 255 indicating a lightest (e.g. white) greyscale level obtainable with the cell. In the electrowetting cell example, the 0 greyscale value corresponds with for example a zero voltage applied to the cell so the first fluid may form a layer configuration across a display area of the cell. In contrast, the 255 greyscale value corresponds with for example a non-zero voltage applied to the pixel so the first fluid is in a fully retracted configuration.
The human eye is more sensitive to gradual changes in luminance for darker greyscale levels than for lighter greyscale levels. Therefore, if a luminance value of a cell increases linearly in dependence on increasing greyscale value, this change of luminance is not perceived as a linear change of luminance by a human eye. Therefore, a so-called gamma correction may be applied to change the luminance of a given greyscale level output by a cell, to improve a luminance behavior of the cell, so that a function of luminance value against greyscale value of a cell more closely matches a luminance perception of the human eye. Such gamma correction may be performed by an electronic circuit or a processor modifying an input greyscale level value of a display effect encoded by input image data to an output signal for driving the cell to output a display effect with a gamma corrected greyscale level. Such gamma correction can increase a power demand of a display device.
It is desirable to improve the luminance behavior of an electrowetting element.